Sovereignty Blockade Unit. An anchorable structure used in Nullsec sov warfare. When more than 50% of a system's gates have an SBU anchored on-grid, the system becomes vulnerable to attack. Only one SBU can be anchored per gate. Thus, it's common to see friendly SBUs in a friendly system: this is a defensive maneuver to essentially "steal" the SBU slot on a gate from any invaders for as long as possible.